Characteristics of chitin-binding proteins from Streptomycetes

During growth in the presence of chitin-containing substrates, many Streptomyces strains have been shown to secrete formerly unknown, small chitin-binding proteins (CHBs) which lack enzymatic activity, specifically target and invade, like a glue, alpha-chitin, but not beta-chitin or other polysaccharides. CHBs were purified, and their N-terminal amino acids were determined. Deduced oligonucleotides were used to identify the corresponding genes, which were then sequenced. The deduced CHB1 and CHB2 proteins contain 201 and 200 amino acids, respectively, 77.7% of which are identical. Several motifs, including the relative location and spacing of four tryptophan residues, are conserved in CHB1 and CHB2. The affinity of CHB1 to crab shell chitin is two times higher than that of CHB2. Comparative studies of various generated mutant CHB1 proteins led to the conclusion that mainly one of the exposed tryptophan residues directly contributed to the interaction with chitin. Using CHB doupled with FITC (fluoresceine isothiocyanate), a highly specific and rapid assay was developed to visualize the location of crystalline alpha-chitin within native samples by fluorescence or confocal laser microscopy. In contrast, the N-terminal domain (12 kDa) of the S. olivaceoviridis exochitinase can be used to detect alpha- and beta-chitin. The structural parameters inducing the recognition and possible loosening of alpha-chitin or of alpha- and beta-chitin are at present being investigated.